BAGHDAD, Iraq – U.S. troops detained the eldest son of Iraq's most influential Shiite politician for nearly 12 hours yesterday as he crossed back from Iran – the same route Washington believes is used to keep powerful Shiite militias flush with weapons and aid.

Even though the U.S. ambassador issued a rapid apology, the decision to hold Amar al-Hakim risks touching off a backlash from Shiite leaders when their cooperation is needed most to keep a major security sweep through Baghdad from unraveling.

It also highlights the often knotty relationship between U.S. military authorities and Iraq's elected leaders, whose ties to neighboring patrons – Syria backing Sunnis, and Iran acting as big brother to majority Shiites – add fuel to sectarian rivalries and bring recriminations from Washington about alleged arms smuggling and outside interference.

Shiite reaction to the detention was quick and sharp, with some officials suggesting it was a veiled warning about the limits of ties to Iran.

“What happened is unacceptable,” Shiite lawmaker Hamid Majid Moussa told Al-Forat television. “The Iraqi government and the American forces must put an end to such transgressions.”

The station is just one part of the multilayered clout of the al-Hakim family.

Al-Hakim's father, Abdul-Aziz al-Hakim, met with President Bush at the White House in December. He is the leader of the Supreme Council for the Islamic Revolution in Iraq, or SCIRI, the country's largest political force.

The bloc carries the strongest voice in the 275-seat parliament and holds crucial sway over the fate of Prime Minister Nouri al-Maliki. It also maintains close ties to Iran, which hosted the elder al-Hakim and other SCIRI officials before the fall of Saddam Hussein's government in 2003.

In December, U.S. forces seized two Iranian security agents at the elder al-Hakim's compound in Baghdad. Six other Iranians were arrested Jan. 11 at an Iranian liaison office in northern Iraq. The U.S. military said they were members of Iran's elite Revolutionary Guard. Tehran has denied the allegation.

Washington has repeatedly accused Iran of funneling weapons to militants, including lethal roadside bombs that have targeted U.S. troops.

But U.S. ambassador to Iraq Zalmay Khalilzad tried to defuse any showdowns with Shiites that could upset a 10-day-old offensive seeking to reclaim Baghdad's streets from militants and sectarian death squads. Shiite militias appeared to clear the way for the effort by rolling back fighters and checkpoints.

“I am sorry about the arrest,” Khalilzad said. “We don't know the circumstances of the arrest and we are investigating and we don't mean any disrespect to Al-Sayed Abdul-Aziz al-Hakim or his family. We will find out what has happened.”

A U.S. Embassy spokesman in Baghdad, Lou Fintor, said al-Hakim “was not singled out” and “soldiers were following standard procedure” since the border crossing was closed at the time. But he did not offer details on how al-Hakim's entourage entered Iraqi territory if the crossing point was sealed.

The younger al-Hakim, 35, was taken into custody at the Zirbatyah crossing point southeast of Baghdad along with his security guards, said his father's secretary, Jamal al-Sagheer. Al-Hakim was freed about 12 hours later, but his bodyguards remained in custody, al-Sagheer said.

The New York Times quoted advisers to al-Hakim as saying U.S. forces had beaten several of the guards after stopping the convoy. The Times also quoted an unidentified U.S. military official as saying al-Hakim was detained because he had an expired passport and was traveling with people who had a large number of guns.

But in an interview after his release at the provincial governor's office in Kut, al-Hakim displayed a passport with an expiration date of Sept. 17, 2007, The Times reported on its Web site.

“They arrested me and my guards in an unsuitable way, and they bound my hands and blindfolded me,” The Times quoted Amar al-Hakim as saying. “They took our phones, bags, money, documents and the guards' weapons, and sent us to an American base.

“They claim the reason for the arrest was because my passport had expired,” he said, “but as you can see my passport expires on the 17th of September.”

U.S. officials could not immediately be reached for comment on the report, because of the late hour in Baghdad.

Amar al-Hakim heads a charity dedicated to the memory of his uncle, Grand Ayatollah Mohammed Baqir al-Hakim, who was killed along with scores of others in a car bombing in Najaf in August 2003. His father took over SCIRI after the killings, and Amar is apparently being groomed to take his place someday.